1. Field of the Invention
This invention relates generally to the field of wireless communications and, more particularly, to the introduction of hysteresis into the hard handoff of a mobile station between cells, in response to measuring round trip delay in communications with the mobile station.
2. Description of the Related Art
As is well understood in the code division multiple access (CDMA) IS-95A/B and IS-2000 standards, a so-called soft handoff permits simultaneous transmissions between a mobile station and two or more base stations or cells. Thus, as a mobile station communicates in a boundary area between cells, a hard handoff decision need not be made to transfer the mobile station communications exclusively to either one cell or the other. Hard handoffs, such as used in frequency division multiple access (FDMA) as described by the advanced mobile phone service (AMPS, EIA/TIA-553), often result in a system instabilities. In the AMPS system, a decision to handoff is based upon signal strength. When a base station detects signal strengths below a predetermined threshold in communications with a mobile station, the system locates another base station where the mobile station signals are detected at a level above the threshold, and the mobile station""s call is transferred to that base station.
Because of changes in signal strength due to the mobile station""s change of position or obstacles in the signal path, the system may direct that the mobile station undergo a series of hard handoffs in a short period of time. This xe2x80x9cping-pongxe2x80x9d effect increases system overhead and often results in a call to the mobile station being dropped. The soft handoff largely eliminates the problems associated with having to make such a hard handoff decision.
However, even in CDMA communications, hard handoffs are still required in some situations, such as when the mobile must change from CDMA to analog modulation, or when the mobile station crosses a boundary between two different carriers (service providers). As is well known in the art, there are two fundamental methods of managing a hard handoff in CDMA communications; using pilot symbols or round trip delay (RTD) to make the transfer decision. In the pilot symbols method, the mobile station measures the signal strength of the pilot symbols transmitted by neighboring base stations. The signal strength of the pilot symbols are compared, and handoff decisions are made by the system in response to these measurements. However, this method requires additional transmitter equipment to generate the pilot symbols, mobile station processing time to make the measurements, and overhead to communicate the measurements to the system through the base stations.
The RTD system requires that the base stations measure the RTD of their communications with a mobile station. An assumption is made that the base station with the smallest RTD is closest to the mobile station. Using the RTD measurements of the neighboring base stations, the system can make a determination of which base station is most likely to remain in communication with the mobile station, and a hard handoff results from this analysis.
FIG. 1 illustrates a hard handoff based upon an RTD triggering mechanism (prior art). When a mobile station is in the traffic state (a call is in progress), a hard handoff is triggered as soon as the RTD measurement among the active CDMA channels is greater than a predetermined RTD threshold.
The mobile station is shown being served by a CDMA channel {Cell1,F1} right before the trigger. The hard handoff is triggered as soon as the mobile station""s RTD crosses the RTD threshold for CDMA channel {Cell1,F1}. The CDMA channel that causes the hard handoff trigger is called the trigger CDMA channel. In this case, the CDMA channel {Cell1,F1} is the trigger CDMA channel and the system uses the information in the target list of this CDMA channel to determine the next step in hard handoff processing that is hard handoff type determination.
The RTD method of triggering advantageously requires no additional equipment. The disadvantage is that inaccuracies in RTD measurement may cause premature or late triggering. To achieve the required hard handoff reliability with this triggering mechanism, the system must be designed with more overlap between border sites for cross-carrier hard handoff. Alternately, the system can trigger sooner, which implies not using the capacity and coverage of the border CDMA channels to their full extent.
FIG. 2 illustrates a well known problem associated with the use of the RTD hard handoff triggering method (prior art). It is assumed that the RTD threshold from a second base station 10 (RTD_TH2) is equal to one-half times the RTD threshold of a first base station 12 (RTD_TH1). That is, 2(RTD_TH2)=(RTD_TH1). For a mobile station 14 moving from Site-1, associated with first base station 12, to Site-2, associated with the second base station 10, the hard handoff triggers when 2xcfx841=RTD_TH1. At a time right before the hard handoff (HHO) trigger, the mobile station 14 transmits the signal S(t) at S(txe2x88x92RTD_TH1/2). At a time right after the HHO trigger, the mobile station 14 transmits the signal Sxe2x80x2(t) at Sxe2x80x2(txe2x88x92RTD_TH1/2). Despite the handoff from first base station 12 to the second base station 10, the mobile station 14 has timed its return communication with a delay associated with the receipt of communications from the first base station 12. Site-2 (second base station 10) receives the signal Sxe2x80x2(txe2x88x92RTD_TH1/2) at Sxe2x80x2(txe2x88x92(RTD_TH1 /2+xcfx841)). As a result, the measured Round Trip Delay at Site-1 is equal to RTD_TH1/2+xcfx841. Since 2(RTD_TH2)=RTD_TH1, the measured RTD at Site-2 is equal to (RTD_TH2+xcfx841). Since this value is larger than the RTD threshold for Site-2, (RTD_TH2), there is another trigger. This ping pong effect is not desirable.
The cause of the timing problem is the mobile station""s time reference slewing. That is, the mobile does not change its time reference from Site-1 to Site-2 instantaneously. Even though the mobile station receives a communication from Site-2 at an earlier time, the mobile stations reply message is delayed as if the mobile station was replying to Site-1. Consequently, Site-2 perceives the mobile station as being further away than it actually is. The IS-95 and IS-2000 standards state that the time reference of the mobile station shall slew at a rate between 3/8 and 10/8 chips per second, where a chip refers to one bit in pseudorandom noise (PN) spreading code which occurs at the rate of 1.2288 mega chips per second (Maps).
It would be advantageous if the RTD method of performing a hard handoff included a hysteresis mechanism to be used while the mobile station system clock is slewing, following a hard handoff.
It would be advantageous if the mobile station""s system clock slewing could be accounted for in determining a hard handoff using the RTD decision process.
It would be advantageous if hard handoffs were permitted as quickly as possible after the mobile station system clock stopped slewing.
In a CDMA cell base station subsystem, a method is provided for introducing hysteresis in a call handoff. Such a method can be implemented as software and machine-executable instructions which are stored in RAM, optical, electrical, or magnetic storage mediums. The method comprises: detecting instability in a mobile station (MS) system clock; and, in response to detecting the mobile station clock instability, suspending hard handoffs of the mobile station.
The hysteresis is introduced following the hand off of the mobile station from a first cell to a second cell. A base station in the second cell detects the mobile station system clock instability by measuring the slew rate of the mobile station system clock. Measuring the slew rate of the mobile station system clock includes: measuring the round trip delay (RTD) of a first message at a first time, from the second cell base to the mobile station; measuring the RTD of a second message at a second time, from the second cell base to the mobile station; calculating the difference between RTD measurements; and, calculating the RTD rate of change. Detecting the mobile station clock instability includes: setting a predetermined RTD rate of change threshold; and, suspending handoffs when the RTD rate of change calculation exceeds the RTD rate of change threshold.
Setting the RTD rate of change threshold also includes estimating the first slew rate of the mobile station. Then, detecting instability in the mobile station system clock includes: in response to calculating the RTD rate of change, calculating the mobile station measured slew rate; and, suspending the handoff of the mobile station includes suspending the handoff when the measured slew rate exceeds the first slew rate estimate.
A CDMA system for introducing hysteresis into call handoff is also provided. The system comprises a mobile station having a system clock for initiating message transmission in response to receiving messages. A cell base station includes a clock to measure message timing, a handoff circuit to negotiate mobile station handoffs with other cells, and a detector connected to the clock to detect instabilities in the mobile station system clock. The detector is connected to the handoff circuit, to suspend handing off the mobile station when instabilities are detected in the mobile station system clock.
The base station detector measures the slew rate of the mobile station system clock by measuring the RTD between messages. The detector calculates the difference between RTD measurements, and calculates the RTD rate of change. The cell base station detector signals the handoff circuit to suspend a handoff of the mobile station when the RTD rate of change calculation exceeds a predetermined RTD rate of change threshold. The mobile station system clock changes the system time at a first predetermined slew rate, and the RTD difference threshold includes accepting a first slew rate estimate. The detector measures the mobile station slew rate in response to calculating the RTD rate of change, and signals the handoff circuit to suspend a mobile station handoff when the measured slew rate exceeds the first slew rate estimate.